WPAN [Wireless Personal Area Network] networks are known for some years; a PAN [Personal Area Network] network can be defined as a computer network for communicating among devices close to one person; a WPAN network is a PAN network using wireless short-range communication technologies.
A communication technology which is very often used for implementing a WPAN network is ZigBee.
One of the main and recent applications of WPAN networks is WSN [Wireless Sensor Network] networks.
In a WPAN network the key components are the nodes of the network, also called devices. In general, a WPAN network may comprise a mixture of mains powered devices and battery powered devices; battery powered devices are designed to limit their power consumption so to assure a long lifetime to their batteries. Providing efficient use of energy in WSN networks is particularly important in order to achieve long-term deployment of applications since the sensor network nodes may not be easily recharged or replaced when the energy of their battery is over.
The component of a node of a WPAN network which is primarily responsible for power consumption is the radio transceiver (both when it transmits and when it receives); the typical way of reducing power consumption in an asynchronous WPAN network (a network wherein the nodes do not have a synchronized clock and therefore do not transmit and receive synchronously) is to use “duty-cycling”, i.e. to let the radio transceiver of the devices operate intermittently for short intervals of times; in this way, the operation of each node is a periodic (fixed transceiver operation period) sequence of (short) awake intervals and (long) sleep intervals. Of course, this complicates the communication protocols used in WPAN networks.
From the prior art, there are also known asynchronous WSN networks wherein all (or almost all) the nodes are battery powered and therefore are designed to limit power consumption and wherein special MAC protocols are used to limit power consumption of the radio transceivers.
The article by M. Buetter et al, “X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks”, SenSys 2006, 1-3 Nov. 2006, Boulder, Colo., USA, describes in detail one of such MAC protocols called “X-MAC” based on a fixed-length preamble. According to this protocol, when a source node has to transmit an information packet, its transmitter transmits a series of short and fixed-length preambles, each containing the address of the destination node; small pauses between preambles permit the receiver of the destination node to awake (according to its own internal operation schedule) and to send an acknowledgment that stops the sequence of preambles and signals the availability of the destination node to receive data, i.e. the payload of information packet; non-destination receivers which overhear the strobed preambles can go back to sleep immediately, rather than remaining awake for receiving data.
The article by A. El-Hoiydi and J. Decotignie, “WiseMAC: An Ultra Low Power MAC Protocol for Multi-hop Wireless Sensor Networks”, in Proceedings of the First International Workshop on Algorithmic Aspects of Wireless Sensor Networks, Lecture Notes in Computer Science, LNCS 3121, pp. 18-31, Springer-Verlag, July 2004, describes in detail another of such MAC protocols called “WiseMAC” based on a variable-length preamble. According to this protocol, the operation schedule of a node's direct neighbours is first learned and then used for minimizing the size of a variable-length preamble of an information packet to be transmitted; acknowledgments are not only used to signal the reception of a data packet by a destination node, but also to inform the source node of the remaining time until its next wake-up instant; in this way, a node can keep a table of schedule time offsets of all its usual destinations up-to-date; using this information, a node may transmit an information packet just at the right time, with a preamble of minimized size. According to this article, the duration of the preamble must cover the potential clock drift between the clocks at the source node and at the destination node; this drift is proportional to the time since the last acknowledgement was received by the source node, i.e. since the last transmission from the source node to the destination node; the required duration TP of the preamble is given byTP=min(4θL,TW)wherein θ is the frequency tolerance of the quartz used for generating the clock signals at the nodes, L is the time interval between transmissions, TW is the fixed period of the schedule of the nodes of the network and “min” is a function that determines the “minimum”.